mechanical secondaries vs vacuum secondaries
Race Director
Joined: Jan 2000
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From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
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Nothing is going to hurt the trans. If you want it simple and just bolt it on and forget it..then stick the vacuum one on there.
Just don't be surprised when your buddy buys a Dp and spends a couple of hours at the most making it perfect and runs off and leaves you!
JIM
Just don't be surprised when your buddy buys a Dp and spends a couple of hours at the most making it perfect and runs off and leaves you!
JIM
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
like i mentioned earlier and like alot of others said your border line and both will work just fine.....but upgrade your stall to a 2500 or go to a larger cam etc and that DP will pay big dividends.
i have just one more thing to say , if i had just 1 dollar for every time i heard someone with a vacum secondary carb say " IM NOT SURE IF MY SECONDARIES ARE WORKING'".....i would be a rich man by now.
i have just one more thing to say , if i had just 1 dollar for every time i heard someone with a vacum secondary carb say " IM NOT SURE IF MY SECONDARIES ARE WORKING'".....i would be a rich man by now.
Pro
Joined: Jul 2001
Posts: 704
Likes: 10
From: Blairsville Ga
I don't think my secondaries! Really, I don't believe they are. I tried the paper clip trick and nothing. If I could go back I'd purshase the manual then you don't have to worry about the stupid vacuum hose. Good luck
Le Mans Master
Joined: Oct 2007
Posts: 7,439
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From: Tennessee/Rhode Island
On Holleys with vacuum secondaries, just do a before and after road test. All you have to do is to remove the vacuum hose and plug it that goes to the secondary vacuum diaphram-and the secondaries will NOT open. Is this hard???
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
The vacuum port is internal. There is no hose.
Race Director
Joined: Jan 2000
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From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
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Back in my *street racing* days...I often would completely remove the little throttle link between the primary and secondaries that actually holds the secondaries closed and would put it in my pocket. That convinced most folks that didn't know how they worked that I'd only be running on *2bbls*. Just feather into it...and a little down track go WOT and the secondaries flopped right open. Folks were just flat amazed how fast a 2BBL could run!@!
He he he......
JIM
He he he......
JIM
Le Mans Master
Joined: Oct 2007
Posts: 7,439
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From: Tennessee/Rhode Island
Le Mans Master
Joined: Jun 2008
Posts: 5,284
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From: Rockledge FL
just keep going down in spring tension on the secondaries and when you feel it bog it is opening too soon.
then go back up one.
once open a 750 vacuum and a 750DP with the same jets are going to flow the same air and fuel.
then go back up one.
once open a 750 vacuum and a 750DP with the same jets are going to flow the same air and fuel.
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Intermediate
Joined: Dec 2010
Posts: 44
Likes: 0
From: Fenwick West Virginia
Vacuum vs Mechanical
If you run into bog, your carb is too large for your engine or you do not
have it set up correctly.
HillBilly John...
Le Mans Master
Joined: Jun 2008
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From: Rockledge FL
Burning Brakes
Joined: Jan 2006
Posts: 1,072
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From: Clear Lake Shores Texas
2016 C3 of Year Finalist
Several years ago I put a 670 Street Avenger on my stock (with the exception of a 71 LT1 intake) 69 350/350 and never liked the way the secondary's worked. At least the springs were easy to change.
I replaced it with a 650 DP and couldn't be happier. As others have mentioned it is nice to be able to control the secondary action although I realize it is ultimately no faster.
FWIW I put the electric choke and linkage from the 670 SA on the 650 DP and it worked out great. The fast idle works perfectly and it starts instantly cold or hot.
I also used a reproduction LT1 fuel line and fuel block and it looks factory.
I replaced it with a 650 DP and couldn't be happier. As others have mentioned it is nice to be able to control the secondary action although I realize it is ultimately no faster.
FWIW I put the electric choke and linkage from the 670 SA on the 650 DP and it worked out great. The fast idle works perfectly and it starts instantly cold or hot.
I also used a reproduction LT1 fuel line and fuel block and it looks factory.
Thread Starter
Racer
Joined: Apr 2008
Posts: 319
Likes: 7
From: Rochester NY
It sounds like many people on the forum have had good luck with 650 or 750 DP's on 4 speed cars - which is what I expected - but I have not heard many who have a DP with an auto - anyone out there with an auto and a DP carb - I'm interested in hearing from you.....
Thanks
Thanks
Le Mans Master
Joined: Oct 2000
Posts: 8,897
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From: Santa Ana CA
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
i have a demon 750 dp on both these motors..
1-10.5 350, roller cam,air gap,fastburn heads,3000 stall....no bog
2-11.1 383,hydraulic cam,eliminator,fastburn heads,2500 stall....no bog except in pants
with dp you KNOW that fuel is going in !!
1-10.5 350, roller cam,air gap,fastburn heads,3000 stall....no bog
2-11.1 383,hydraulic cam,eliminator,fastburn heads,2500 stall....no bog except in pants
with dp you KNOW that fuel is going in !!
Race Director
Joined: Dec 1999
Posts: 11,094
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From: Sierra Foothills CA
It sounds like many people on the forum have had good luck with 650 or 750 DP's on 4 speed cars - which is what I expected - but I have not heard many who have a DP with an auto - anyone out there with an auto and a DP carb - I'm interested in hearing from you.....
Thanks
Thanks
Race Director
Joined: Jan 2000
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From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
C2 of Year Winner (performance mods) 2019
2017 C2 of Year Finalist
It's all in the tune. I ran a DP on a 396 Nova with a single plane manifold and 3.08 gears and a 2200 converter. I actually played with it to NOT be so responsive and get right on the borderline of bogging so I could get it to hook decent on street tires.
It's all in the accelerator pump tuning and ignition timing.
Again, VS are safe, I just don't like the way they drive as much.
JIM
It's all in the accelerator pump tuning and ignition timing.
Again, VS are safe, I just don't like the way they drive as much.
JIM
Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
It sounds like many people on the forum have had good luck with 650 or 750 DP's on 4 speed cars - which is what I expected - but I have not heard many who have a DP with an auto - anyone out there with an auto and a DP carb - I'm interested in hearing from you.....
Thanks
Thanks
The engine that came in the 75 is a milder 406 making 545 hp with a Demon 750 DP. As it came to me, it did not bog. It also would just roast the tires when you stomped it. It got lousy gas mileage though. When this engine goes into my 64 SS I will tune it so it performs excellent and gets good mileage.
If a DP carb is matched to the build and is tuned properly, it will not bog, it will run great, and you can get good mileage. I've ran many DP's with autos.